In recent years, liquid crystal has been used for a variety of devices; in particular, a liquid crystal display device (liquid crystal display) having features of thinness and lightness has been used for displays in a wide range of fields.
As a method for applying an electric field to a liquid crystal included in a liquid crystal display device, a vertical electric field mode and a horizontal electric field mode can be given. As a horizontal electric field mode of a liquid crystal display panel, there are an in-plane switching (IPS) mode in which a pixel electrode and a common electrode are provided without overlapping with each other and a fringe field switching (FFS) mode in which a pixel electrode and a common electrode overlap with each other with an insulating film provided therebetween.
A liquid crystal display device of an FFS mode has a slit-shaped opening in a pixel electrode, and alignment of liquid crystal molecules is controlled by applying an electric field generated between the pixel electrode and a common electrode to the liquid crystal in the opening.
The liquid crystal display device of an FFS mode has a high aperture ratio, a wide viewing angle, and an effect of improving an image contrast, and has been widely used recently (see Patent Document 1).
In a display device, a displayed image is changed several tens of times per second. The number of times an image is changed per second is called a refresh rate. The refresh rate is also referred to as driving frequency. Such high-speed screen switching that is hard for human eyes to be recognized is considered as a cause of eye strain. Thus, Non-Patent Documents 1 and 2 have proposed that the refresh rate of an LCD is lowered to reduce the number of image rewriting operations. Moreover, driving with a lowered refresh rate enables the power consumption of the display device to be reduced.